Television pick-up tube comprising electrostatic electron-optical means



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Dec. 18, 1956 TELEVISION PICK-UP TUBE COMPRISING ELECTROSTATIC TELEVISION PICK-UP TUBE COMPRISING ELEC- TROSTATIC ELECTRON-OPTICAL MEANS Jan Care] Francken, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, as trustee Application July 8, 1952, Serial No. 297,702

Claims priority, application Netherlands August 9, 1951 Claims. (Cl. 315-11) This invention relates to pick-up tubes comprising electrostatic electron-optical means and devices comprising such tubes.

In television pick-up tubes use is sometimes made of electron-optical means by which the electron-image, obtained from a luminous image by conversion with the aid of a photo-electric cathode, is electron-optically projected on to a screen having the property of accumulating electrical charges, thus producing a so-called chargeimage. Hereinafter such a screen will be termed target. The electron-optically operative fields are produced by means of magnet coils, but, as an alternative, electrostatic fields may be used.

In this latter case the electric fields accelerating the photo-electrons also serve for focussing them on to the target. It has been pointed out that replacement of the magnetic lens-fields entails ditficulties. As a matter of fact, with the use of a flat photo-cathode and an accelerating anode in the form of a co-axial cylinder whose diameter substantially corresponds to that of the photocathode, only a small part of the surface of the photocathode is reproduced with satisfactory accuracy on the target.

According to the invention, a television pick-up comprising a photo-electric cathode and electrostatic electronoptical means projecting the electron-image obtained by means of the photo-electric cathode on to a target is characterized in that the photo-electric cathode and the opposite accelerating anode both have a curved surface, the photo-cathode surface being concave and the operative part of the accelerating anode surface being convex, the accelerating anode having a small aperture for the passage of electrons projecting the photo-cathode image on to the target, the photo-cathode and the operative part of the accelerating anode being substantially concentric, and the spacing between the photo-cathode and the accelerating anode substantially corresponding to the ditference of the radii of curvature of the curved surfaces.

The use of such electron-optical means in a pick-up tube involves two conflicting conditions. The field strength at the photo-cathode must be considerable in order to obtain a sufiicient resolving power and the velocity at which the photo-electrons impinge on the screen should not exceed a given value. In the present case, similarly as in optics, the term resolving power is to be understood to mean the minimum spacing of two just visually discernable details which, in a pick-up tube, means that on scanning the target they will produce separate impressions which are as such discernable in the image of a picture tube.

Both conditions are satisfied by providing that, according to another aspect of the invention, in co-operation with the afore-said electrostatic electron-optical means, an electric field is operative in a pick-up tube between the accelerating anode and the target, said field retarding the electrons to a velocity at which the secondary-emission coefiicient of the target is in the range of the high values.

In order that the invention may be more readily carried into effect it will now be described with reference to 2,774,909 Patented Dec. 18, 1956 F ice the accompanying drawing, given by way of example, which represents one embodiment thereof, and in which the glass tube consists of two circle-cylindrical parts 1 and 2 inter-connected by a funnel-shaped part 3, both parts being closed by curved bottoms 4 and 5. A cylindrical arm 6 of comparatively small diameter connects with the funnel-shaped portion 3, the other end of the arm 6 being closed and the space within the glass wall being evacnated.

The photo-electric cathode 7 is provided on the bottom 4 and represented diagrammatically. The manufacture thereof, which may take place in the usual way, need not be described in greater detail. It consists of a photo-sensitive layer provided on a conductive metal substratum 19, which is so thin as to absorb very little light, the photo-cathode 7 being electrically connected to this metal layer 19.

The bottom 5 carries, on a support 8, the target 9, whose construction is also not described. Use may be made of targets having a secondary-emission surface and those in which depth of penetration of the electrons is utilised. Many different forms of targets are known. They comprise a carrier either consisting of metal or insulating material having a metallized surface. This target and carrier assembly is connected through the metal support 8 to a point of an amplifying circuit-arrangement and earthed by way of a resistor 10.

The arm 6 comprises an electrode-system producing a directed electron beam. As is seen from the drawing, use is made of a cathode 11, a Wehnelt electrode 12 and an anode 13. The beam is directed to the center of the screen 9 and the point of impact can be displaced over the surface of the screen by means of deflection fields (the means producing them are not shown). The beam serves to erase electriccharges on the target element and to convert them into electric current variations which are fed to the amplifying circuit-arrangement with the use of the resistor 10.

This requires the use of a collector which, in the tube shown in the drawing, is provided in the form of a conductive coating 14 on the wall of the tube. The collector attracts secondary electrons dislodged by the directed electron beam scanning the arget. ation of the pick-up tube it is vital, however, that part of the secondary electrons should fall back on other parts of the target. Hence, the collector is connected to earth so that it has substantially the same potential as the support of the target.

The cathode 11 of the electron gun is connected to. a point of negative potential of, say 1000 v. relatively to earth. This potential is chosen to be such that the target has a suitable secondary-emission coefficient at the electron-velocity then occurring.

An optical image of the scene to be televised is projected on to the photo-electric cathode 7. The local density of emission of the photo-electrons corresponds with the local brilliance of the image. The emission-image is electron-opticallyprojected on to the target 9. In order that the photo-emission may reach its saturation value an accelerating electric field is required at the photo-cathode. Owing to a particular shape of the accelerating anode 15 of the tube in accordance with the invention, said electric field is given a definite form so that it also has the electron-optical properties required to obtain a true reproduction of the photo-cathode image on the target. The accelerating anode 15 has a rounded tip 20, the operative part thereof, facing the photo-cathode and is provided with a small aperture 21 for the passage of electrons. The surface of the photo-cathode is concave. Both surfaces have the same center of curvature. The conductive layer 19 alsosurrounds the space between the photo-cathode 7 and-the operative part 20 of-the anode 15. In the present case, the photo-cathode 7 For satisfactory operand conductive layer 19 may be at a negative potential of 1000 to 2000 v. and the accelerating anode may be electrically connected to the collector 14.

The field strength then produced at the photo-cathode is insuflicient for a satisfactory resolving power. It may be pointed out that this resolving power, so far as it is limited by chromatic aberration, may be approached by the expression:

where N is the number of black and white lines, projected separately, per image height c=a constant having the value of 2 to 3.5

h=height of the image V=the field strength of the photo-cathode.

Better results are obtained if the field strength at the photo-cathode exceeds the field strength determining the velocity at which the electrons impinge on the target element. As regards this velocity allowance has to be made for the fact that the secondary emission factor of the target as a function of the energy of impinging photoelectrons has a curve form exhibiting a maximum, the secondary-emission factor decreasing when the aforesaid electron velocity is exceeded.

In the construction shown in the drawing, this difficulty is avoided by separating the accelerating anode 15 from the collector 14. The accelerating anode 15 extends over part of the path travelled by the electrons, the remaining part of this path being surrounded by the collector 14 which is insulated from the accelerating anode 15 and maintained at the average potential of the signal plate of the target element. Hence, the collector 14 serves as a decelerating electrode. In a device comprising this pick-up tube, the accelerating anode 15 is given a positive voltage relatively to the photo-cathode, which voltage exceeds the potential difference between the photo-cathode 1 and the target 9. This voltage depends upon the resolving power to be obtained. A considerable improvement is obtained by applying to the anode a voltage corresponding to 2 to 3 times the voltage set up between the photo-cathode 1 and the collector 14.

In this event, an electron-lens is formed between the accelerating anode 15 and the collector or decelerating electrode 14. By supplying the voltage to the accelerating anode through a control member, the strength of said lens is controllable which permits sharp-adjustment of the image projected on to the target element, thus reducing the dependency on the position of the accelerating anode 15 with respect to the photo-cathode 1.

A further advantage is that an ion spot cannot occur on the photo-cathode 7. The positive ions cannot find their way from the space containing the target 9 to the photo-cathode space, since the accelerating anode is positive relatively to the collector. Positive ions in the photocathode space are strongly diverged towards the photocathode.

What I claim is:

1. A television pick-up tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemi-spherical surface facing said concave cathode, said concave cathode and said convex anode surface being substantially spherically concentric, said anode having a small aperturetherethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and said convex anode surface, a target electrode disposed on the side of said anode remote from said cathode and at the other end of said tube for receiving electrons emanating from said cathode and passing through the aperture in said anode, a decelerating electrode insulated from said anode and surrounding a portion of the space between said anode and target electrode, means for applying a given potential to said cathode, means for applying a potential which is greater than said given potential to said anode to thereby produce an accelerating field between said cathode and anode, and means to apply a potential to said decelerating electrode which is less than the potential applied to said anode to thereby produce a decelerating field between said anode and target.

2. Television pick-up tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex herni-spherical surface facing said concave cathode, said concave cathode and said convex anode surface being substantially spherically concentric, said anode having a small aperture therethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and said convex anode surface, a substantially flat target electrode disposed on the side of said anode remote from said cathode and at the other end of said tube for receiving electrons emanating from said cathode and passing through the aperture in said anode, a decelerating electrode insulated from said anode and surrounding a portion of the space between said anode and target electrode, means for applying a given potential to said cathode, means for applying a potential which is greater than said given potential to said anode, to thereby produce an accelerating field between said cathode and anode, means to apply a potential to said decelerating electrode which is less than the potential applied to said anode to thereby produce a decelerating field between said anode and target, and means for producing and deflecting an electron beam and directing same at the target plate in order to scan the target plate.

3. A television pick-up tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemispherical surface facing said concave cathode, said concave cathode and said convex anode surface being substantially spherically concentric, said anode hav-' ing a small aperture therethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and said convex anode surface, a target electrode disposed on the side of said anode remote from said cathode and at the other end of said tube for receiving electrons emanating from said cathode andpassing through the aperture in said anode, said anode surrounding a portion of the space between the aperture and the target electrode, a decelerating electrode insulated from said anode and surrounding most of the space between said anode and target electrode, means for applying a given potential to said cathode, means for applying a potential which is greater than said given potential to said anode to thereby produce an accelerating field between said cathode and anode, and means to apply a potential to said decelerating electrode which is less than the potential applied to said anode to thereby produce a decelerating field between said anode and target.

4. A television pick-up tube comprising an envelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode elecnected to said cathode and surrounding the space between said cathode and said convex anode surface, a substantially flat target electrode disposed on the side of said anode remote from said cathode and at the other end of said tube for receiving electrons emanating from said cathode and passing through the aperture in said anode,

said anode surrounding a portion of the space between the aperture and the target electrode, a decelerating electrode insulated from said anode and surrounding most of the space between said anode and target electrode, means for applying a given potential to said cathode, means for applying a potential which is greater than said given potential to said anode to thereby produce an accelerating field between said cathode and anode, and means to apply a potential to said decelerating electrode which is less than the potential applied to said anode and greater than said given potential to thereby produce a decelerating field between said anode and target.

5. A television pick-up tube comprising anenvelope, a photo-electric cathode source of electrons having a concave surface at one end of said envelope, an anode electrode disposed adjacent said cathode and having a convex hemispherical surface facing said concave cathode, said concave cathode and said convex anodesurface being substantially spherically concentric, said anode having a small aperture therethrough for the passage of electrons from said cathode, a conductive layer electrically connected to said cathode and surrounding the space between said cathode and said convex anode surface, a substantially flat target electrode disposed on the side of said anode remote from said cathode and at the other end of said tube for receiving electrons emanating from said cathode and passing through the aperture in said anode,

a decelerating electrode electrically connected to said target electrode and insulated from said anode and surrounding most of the space between said anode and target electrode, means for applying a given potential to said cathode, means for applying a potential which is greater than said given potential to said anode to thereby produce an accelerating field between said cathode and anode, and means to apply a potential to said decelerating electrode which is less than the potential applied to said anode and greater than the potential applied to said cathode to thereby produce a decelerating field between said anode and target, the potential difference between said anode and said decelerating electrode being between about 2 and 3 times the potential difference between said cathode and said decelerating electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,172,728 Bruche Sept. 12, 1939 2,286,280 Iams June 16, 1942 2,396,023 Schantz Mar. 5, 1946 2,613,330 Bruining et al Oct. 7, 1952 

